Automotive automatic transmissions typically have a plurality of components including torque convertors and clutches wherein states of engagement and disengagement must be controlled. Initially such components were controlled with fluid pressure logic components. In an era approximately beginning in the 1980's, more and more automotive transmissions have been brought forth wherein the control of the torque convertor and clutches has been taken over by electronic logic controls which utilize solenoid valves to control the hydraulic control fluid which actuates or relieves the operation of the various fluid pressure actuated components of the transmission. Currently, many of these solenoid valves are hermetically sealed within the transmission fluid. Accordingly, the solenoid actuated valves are submerged within the hydraulic fluid.
Most solenoid valves utilized in vehicle automotive transmissions have a coil wrapped in a non-magnetic bobbin. Encasing the coil and providing a portion of the magnetic loop of the solenoid valve is a casing. On an interior diameter of the bobbin is typically a flux tube and a pole piece. Slidably mounted inside the flux tube and pole piece is an armature which contacts a valve member. To align the flux tube and pole piece and to maintain a close axial separation between the flux tube and pole piece, there is provided a non-magnetic material alignment tube. An example of such a solenoid valve can be found by a review of PCT Patent Application No. PCT/US2010/021924.
Due to manufacturing variations, the current/force operation of individual solenoid valves can vary slightly. To achieve the optimum performance in a transmission, it is desirable that the solenoid valves be calibrated. Previously, calibration was done mechanically, but it has been found preferable that calibration of the solenoid valves be achieved utilizing a software methodology as revealed in U.S. Pat. No. 7,474,948. When the solenoids are calibrated electronically, it is desirable that the operation of the solenoids be consistant over the lifetime of its operation within the transmission as much as possible. As previously mentioned, solenoids in automotive transmissions are often submerged within the transmission fluid. During operation of the vehicle and due to wear of the various gears, shafts and bearings of the transmission, metallic particles are generated as contaminants in the transmission fluid. These contaminants can often become lodged within the solenoid valves since they are attracted to the magnetic fields which permeate the solenoid valves. As mentioned previously, most solenoid valves have a casing and a separate flux tube and pole piece. Where there are interfaces between the various components, over process of time, metallic components can become lodged within the interfaces between the various components. When metallic components become lodged within the interface of the various components, the flux efficiency or density of the various components can be modified thereby changing the operation of the solenoid valve away from its initial calibrated setting. It is desirable to provide a solenoid valve which especially in submerged environments will have a more constant operation over time regardless of the presence of metallic contaminants from the hydraulic fluid that the solenoid valve is submerged within.